In recent years, inkjet recording apparatuses that form images by an inkjet recording system are widely used as image forming apparatuses since high-definition image qualities that are comparable to those of silver halide photographs can be obtained due to rapid advances in recording technologies.
In such process for forming an image using an inkjet recording apparatus, image formation at a desired image density, high-speed printing, suppression of strike-through (a phenomenon in which a printed ink passes through a recording medium and the formed image is seen from the rear surface) during printing, suppression of color bleeding between inks having different colors, and suppression of occurrence of image pollution due to offset, and the like are required, and various studies have been done in order to solve these problems.
For example, in order to solve the problem of strike-through, suggested is use of an inkjet ink that is characterized by that, when the Bristow measurement is conducted for a plane paper and the measured values are plotted, the plots show liquid transition amounts L (ml/m2) as a vertical axis and (γ·t/η)1/2 as a horizontal axis, the value of the gradient of the graph is less than 0 to 0.9-fold of the value of a gradient when pentadecane is used as a measurement liquid. γ shows the surface tension (mN/m) of the inkjet ink, t shows the contact time (msec), and η shows viscosity (mPa·sec).
However, in the case when an image is formed by the above-mentioned inkjet ink, the problem of strike-through is solved, but the penetration of the inkjet ink into the recording medium may be suppressed in order to solve such problem. Therefore, the problems of suppression of color bleeding and occurrence of image pollution due to offset are not solved even by the above-mentioned inkjet ink. Furthermore, in the process for forming an image using the above-mentioned inkjet ink, printing is conducted at a high-speed in many cases when an inkjet recording apparatus of a line head system is used, and thus color bleeding and occurrence of image pollution due to offset may be significant.
It is considered that it is effective for suppressing color bleeding to form an image by ejecting inks of two or more colors in the decreasing order of the penetrabilities of the inks into a recording medium, and for example, the following processes for forming an image have been suggested. Specifically, a process for forming an image by ejecting inks in the decreasing order of the contents of a penetrability imparting agent, and a process for forming an image by ejecting color inks each having a fast penetration velocity into a recording medium prior to the ejection of a black ink having a slow penetration velocity have been suggested.
Furthermore, as a process for suppressing strike-through and color bleeding when an image is formed by an inkjet recording apparatus of a line head system, a process including forming an image by ejecting inks each containing an organic solvent having an SP value in a specific range which provides an ink transition amount after 0.05 seconds of a contact time by the Bristow method of 15 ml/m2 or more in the decreasing order of the ink transition amounts is exemplified.
However, the penetrabilities of the inks which are considered in the above-mentioned processes for forming an image including ejecting inks of two or more colors in the decreasing order of the penetrabilies into the recording medium are the penetrabilities of the inks into the recording medium from 10 seconds after the printing. Therefore, in these processes for forming an image, color bleeding cannot be suppressed sufficiently by an inkjet recording apparatus of a line head system by which high-speed printing is conducted, for example, under such a condition that the ejection interval of the inks between the adjacent line heads becomes 1 second or less.
Furthermore, in the above-mentioned process for forming an image in which strike-through and color bleeding are suppressed when an image is formed by an inkjet recording apparatus of a line head system, the effect of suppressing strike-through is not sufficient, and thus further improvement is demanded. In addition, in this process for forming an image, since inks having excellent penetrabilities into a recording medium are used, the pigments included in the inks easily enter the inside of the recording medium in accordance with the penetration of the inks. Therefore, an image having a desired image density is difficult to be formed in this process.
Furthermore, the above-mentioned four conventional image forming processes do not consider suppression of image pollution due to offset which occurs by rubbing of a formed image by a roller disposed on an ejection unit when the recording medium on which the image has been formed is discharged from the inkjet recording apparatus. Therefore, in these image forming processes, image pollution due to offset may easily occur depending on the compositions of the inks.